1. Field of the Invention
The present invention relates to a semiconductor device having a silicon substrate sliced off from a silicon ingot produced by a pulling method (Czochralski method) or floating zone method (FZ method) and to a method of producing the same.
2. Description of the Related Art
Nowadays, semiconductor wafers produced by the Czochralski's pulling method are being widely used for silicon substrates of semiconductor device. These semiconductor wafers are hereinafter referred to as CZ waters. When silicon is pulled out from a quartz crucible, oxygen is dissolved therein. Thus, the CZ wafers contain supersaturated oxygen. The excessive oxygen atoms are precipitated as for example SiO.sub.2 in a heating step of the semiconductor device production process.
It is said that the oxygen atoms contained in the silicon as interstitial atoms contribute to improving the mechanical strength of the CZ wafer and that the precipitation for example of SiO.sub.2 allows metal impurities harmful to semiconductor device to be removed therefrom. Thus, when CZ wafers are produced, the concentration of interstitial oxygen contained in silicon is properly controlled.
However, when oxygen is precipitated in the vicinity of the device forming surface (active region) of the CZ wafer, crystal defects such as OSF (Oxidation induced Stacking Fault) or defects of oxide films laminated on the CZ wafer result, which adversely affect the device on the CZ wafer.
To prevent oxygen atoms from precipitating in the vicinity of the device forming surface, the concentration of interstitial oxygen in the vicinity of the front surface of the silicon substrate is lowered so that oxygen atoms do not precipitate.
However, even if the concentration of interstitial oxygen in the vicinity of the front surface of the silicon substrate has been lowered so that oxygen atoms do not precipitate (for example, the concentration of oxygen on the outermost surface is 7.times.10.sup.17 cm.sup.-3 in former old ASTM), the distribution of concentration of interstitial oxygen in the depth direction of the substrate indicates that the concentration of interstitial oxygen of the device active region and an inner region (with a depth of approximately 10 .mu.m from the front surface) which affects the device operation is 1.times.10.sup.18 cm.sup.-3 or more. Thus, although the density of oxygen atoms is low, there are numerous interstitial oxygen atoms which can precipitate.
In the device forming process, when metal impurities such as Fe intrude into a silicon substrate, they cause the critical concentration of the precipitation of oxygen to lower. Thus, the above-mentioned concentration of oxygen is insufficient to prevent the precipitation of oxygen. Therefore, the concentration of oxygen should be sufficiently lowered both on the outermost surface of the silicon substrate and in an inner region with a depth which affects the device operation.
With respect to a so-called expitaxial wafer where a single-crystal silicon semiconductor layer is equally and expitaxially grown on the front surface of a silicon substrate, the concentration of oxygen on the epitaxial layer is so low as to prevent oxygen from precipitating. However, the production cost of the epitaxial wafers is high. In addition, the production of these wafers requires advanced technologies.
Moreover, when the concentration of interstitial oxygen of a wafer is low, its withstand resistance against thermal stress decreases. Thus, the strengths of the expitaxial wafer and the FZ wafer are lower than that of the CZ wafer. As a result, in the expitaxial wafer and the FZ wafer, dislocation and defects such as slip tend to occur, which adversely affect the device operation.